1. Field of the Invention
The present invention relates to a printing apparatus, for example to an ink jet printing apparatus which prints by discharging a liquid such as ink.
2. Description of the Related Art
In ink jet printing apparatuses, to realize a high-quality image, a high conveyance accuracy of a sheet-like printing media (in the present specification, simply referred to as a “sheet”) is required. Recently, to control conveyance with a higher degree of accuracy, direct sensors are being realized which directly detect a movement amount of the sheet by imaging the sheet surface and then performing image processing. For example, U.S. Pat. No. 7,104,710 discusses a technique for controlling conveyance by using a direct sensor. In the apparatus discussed in that document, the direct sensor is provided on a carriage which mounts a print head, or at a position facing a discharge port face of the print head.
However, in this configuration the sheet can only be imaged at a position where the direct sensor is fixed in the sheet conveyance direction. Therefore, during sheet conveyance, the sheet may not be present at a sensing position of the direct sensor in a certain period (hereinafter, referred to as “sensing disabled period”). For example, in a case where an image is printed by a multipass method during printing on a trailing edge or a leading edge of the sheet, if the sheet edge portion deviates from the sensing position during printing, thereby making sensing impossible, conveyance cannot be controlled with a high degree of accuracy. As a result, there is the problem that the quality of that portion cannot be guaranteed.
In addition, in this configuration, if the direct sensor provided on the carriage is single, it is difficult to obtain a detection region exceeding the movable stroke of the reciprocating carriage, and there is the possibility of being impossible of covering the maximum sheet width. If the direct sensor is provided to be offset to the side of one end (e.g. reference side) of the moving direction on the carriage, the very end of the sheet can be read when the carriage moves to the side of the end. However, when the carriage moves to the other end (non-reference side), it is impossible to read the very end of the sheet with the direct sensor.